A number of forms of ink jet printer have been used to apply ink compositions to a wide range of substrates. In many cases the ink is put up in the form of a solution-or suspension in an organic solvent, notably MEK. The use of MEK provides an ink which is highly suitable for use in ink jet printers, since it gives an ink which can readily be jetted, which is aggressive to many plastic substrates so that the printed image adheres firmly to the substrate, and which dries rapidly so that the printed image resists smudging or smearing allowing the printed substrate to be handled almost immediately after printing. Furthermore, MEK readily dissolves most of the dyestuffs and other components used in ink compositions, giving the printer the freedom to use a wide range of ink compositions.
Despite the many technical advantages in the use of MEK and similar solvents, major problems arise in that such solvents are often toxic or present health hazards and are becoming less and less acceptable from an environmental standpoint. Thus, MEK is flammable and presents a health hazard, notably when vapours are inhaled or fluid contacts the skin, and it would be desirable to use a solvent or carrier medium for the ink which is physiologically acceptable, notably so as to allow the ink to be applied directly to a foodstuff or to packaging closely associated with a foodstuff.
It has therefore been proposed to use lower alkanols or water as the solvent or carrier medium for the ink. Whilst such solvent or carrier media reduce or avoid the physiological problems associated with the use of MEK, ink formulations using such a solvent or carrier dry slowly and spread upon contact with non-porous surfaces, notably many of the clay filled glossy surfaced papers currently used in the packaging industry, so that the printed image smears and loses its sharp definition. Furthermore, since many of the dyestuffs or other components required for the ink compositions are often insoluble or only slightly soluble in water or alkanols, the use of such solvent or carrier media limits the range of ink formulations the printer can use.
It has been proposed to use ethyl acetate as a carrier medium for pigment based inks in contact printing processes where the inks have a high viscosity, typically in excess of 100 Cps, and contain high levels of suspended pigments and binders. Such compositions are not suitable for use in an ink jet printer, where the ink must have a viscosity of less than 40 Cps and blockage of the fine nozzle orifices would occur due to the high level of solids in such inks.
Despite the fact that ethyl acetate has physiological advantages over the use of MEK, it is relatively non-polar and does not dissolve the salts and many of the dyestuffs now desirable for use in ink compositions for ink jet printers. However, we have found surprisingly that ethyl acetate produces an ink composition which does not spread as much as a corresponding ethanol based formulation when applied to a clay filled paper or other porous substrate.
Furthermore, we have found that mixtures of ethanol and ethyl acetate and their lower alkyl analogues behave synergistically and that the mixtures can be used as the solvent or carrier medium for a wide range of ink compositions. The solvent mixtures enhance the ability of the ethanol to dissolve the desired physiologically acceptable inks even where large proportions of ethyl acetate are present and retain the low spreading property of ethyl acetate. Furthermore, we have also found that the mixture of solvents results in compositions of reduced viscosity, which enhances their jet-ability through the fine orifice apertures of ink jet printers.